This invention relates to multifunctional organo-alkali metal polymerization initiators. Organo-alkali metal compounds have long been used as anionic polymerization initiators. Such initiators first produce what is known in the art as a living polymer. There are three basic methods of carrying out such polymerizations.
In the first, a monomer or a sequence of monomers is simply combined with a monofunctional initiator such as secondary butyllithium and after the desired chain length is achieved the initiator is terminated. In the second, the monomer or a sequence of monomers is contacted with a monofunctional initiator, generally secondary butyllithium, and polymerization is carried out until chains of about 1/2 the desired length are produced after which a difunctional coupling agent is introduced. Alternatively, chains can be 1/3 of the desired length in a trifunctional coupling agent utilized, etc. These processes both give excellent results in commercial scale operations. However, there are times when it would be desirable to have a polymer with a functional group on each end of the polymer chain. With the first process just described, the active lithium entity is only at one end of the growing chain. In the second method just described, the active lithium entities are positioned in the middle of the coupled polymers.
The third method utilizes a multifunctional organolithium initiator such as the diadduct produced by reacting secondary butyllithium with divinylbenzene or substituted divinylbenzenes. These initiators, by virtue of having an active lithium atom at each end grow more or less symmetrically. When the chain reaches the desired lengths, there is an active lithium at each end which can be reacted with the appropriate materials to give functional groups such as OH on each end. This solves the problem of not having an alkali metal atom at each end, but creates new problems. For one thing, the initiators frequently are not soluble in the polymerization solvent. Also the living polymer chain ends seem to associate with each other which causes the polymer to gel as soon as the reaction starts.
It was long ago proposed to prepare multifunctional lithium initiators by reacting an organolithium compound with a polyvinyl silane. While such systems are operable, they give erratic results. For one thing the measured molecular weight tends to be far below the calculated molecular weight indicating that a major amount of the initiator is effectively monofunctional. Also because of the erratic polymerization, there is no uniformity of chain length and thus, an undesirably broad molecular weight distribution results. Finally, as noted hereinabove, the ends of the living chain seem to associate which can result in crosslinking thus giving polymers having higher molecular weight then the target value.